Oil field operators demand access to a great quantity of information regarding the parameters and conditions encountered downhole. A wide variety of logging tools have been and are being developed to collect information relating to such parameters as position and orientation of the bottom hole assembly, environmental conditions in the borehole, and characteristics of the borehole itself as well as the formations being penetrated by the borehole. Among such tools are resistivity logging tools, which measure the electrical resistivity of a formation within a borehole. These tools cause electrical currents to flow within the formations to determine the formation's resistivity. A high resistivity measurement within a porous formation can indicate that hydrocarbons are present in the formation.
The electrical resistivity of a formation is generally anisotropic, i.e., the formation's resistivity will vary depending upon the orientation of an electrical current flowing through the formation. The measurements obtained by a resistivity logging tool may thus vary depending upon the orientation of the current induced in the formation and used by the tool to measure the formation's resistivity. Further, both macro-anisotropy (i.e., anisotropy caused by differing formation layers) and micro-anisotropy (i.e., anisotropy caused by the grains that make up the material of each layer) may both be present. The micro-anisotropy of a given formation layer, however, may not be detectable by resistivity logging tools with measurement resolutions measured in feet or meters, rather than inches or centimeters. Such low resolution tools may thus not fully characterize the anisotropy of the formation at both a micro and a macro level, producing an incomplete and possibly misleading characterization of the formation.
It should be understood, however, that the specific embodiments given in the drawings and detailed description do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.